The present invention relates generally to a method for reducing power required for digital subscriber line (DSL) transmissions, and specifically to a method for reducing power while maintaining a similar signal to noise ratio.
Referring to FIG. 1 a Discrete MultiTone (DMT) communication system is illustrated generally by numeral 100. The system comprises a transmitter 102 and a receiver 150. In the transmitter 102, a set of N Quadrature Amplitude Modification (QAM) symbols cli is used to modulate a set of N tones for every DMT symbol period. The modulation is achieved by feeding the QAM symbols cli, l=0, 1, . . . , N-1, into an Inverse Fast Fourier Transform (IFFT) 104. The index i indicates the DMT symbol period. The QAM symbol cli carries bl bits, where bl, l=0, 1, . . . , N-1, is referred to as a bit allocation table. Further, the QAM symbol cli is scaled by a fine gain gl, l=0, 1, . . . , N-1, before being processed by the IFFT 104. These fine gains are used to change the transmit power in each tone individually. After the IFFT 104, a prefix adder 106 appends a Cyclic Prefix (CP) to the DMT symbol, which is then further conditioned by the transmitter's front end 108 before being transmitted on a line 110. The transmitter's front end includes digital filters 112, a Digital-to-Analog converter (DAC) 114, analog filters 116 and a line driver 118.
At the receiver, the receiver's front end 152 processes the received signal using a analog filters 154, an Analog-to-Digital converter 156 and digital filters 158. A Time Domain Equalizer (TDEQ) 160 partially equalizes the signal at the output of the receiver's front end. A prefix remover 162 remove the Cyclic Prefix and feeds the signal to a Fast Fourier Transform (FFT) 164. At the FFT output, a per-tone single tap Frequency Domain Equalizer (FDEQ) 166 completes the equalization of the signal and a set of slicers 168 retrieves the transmitted QAM symbols cli.
As described above the fine gains are determined by the transmitter. For asymmetric digital subscriber line (ADSL) technology, the fine gains are computed at Initialization after a per-bin signal to noise ratio (SNR) estimation phase, referred to as MEDLEY states. The fine gains are computed together with the bit allocation table. They are used to modify per-tone transmit power in order to minimize and equalize the SNR margin across the tones during data transmission, referred to as SHOWTIME. In particular, given a desired number of bits per DMT symbol B and measured channel SNR profiles, the bit and gain allocation tables bl and gl, l=0, 1, . . . , N-1, are jointly computed such that:
                              B          =                                                    ∑                                  l                  =                  0                                                  N                  -                  1                                            ⁢                              b                l                                      =                                          ∑                                  l                  =                  0                                                  N                  -                  1                                            ⁢                              int                ⁡                                  [                                                            log                      2                                        ⁡                                          (                                              1                        +                                                                              SNR                            l                                                                                Γ                            ·                                                          m                              /                              cg                                                                                                                          )                                                        ]                                                                    ;                            (        1        )                                                                                    log                2                            ⁡                              (                                  1                  +                                                                                    g                        l                        2                                            ⁢                                                                                          ⁢                                              SNR                        l                                                                                    Γ                      ·                                              m                        /                        cg                                                                                            )                                      ⁢                                                  ⁢                          is  an  integer  between                        ⁢                                                  ⁢                          b              min                        ⁢                                                  ⁢                          and                        ⁢                                                  ⁢                          b              max                                ;                ⁢                                  ⁢                  and                                    (        2        )                                          ∑                      l            =            0                                N            -            1                          ⁢                              g            l            2                    ⁢                                          ⁢                                    is  minimized                        .                                              (        3        )            
In the above-equations, Γ is a SNR gap for uncoded QAM modulations (Γ=9.759 dB), m is an imposed system margin, cg is a coding gain for a given coding schemes used by the transmission and SNRl is the Signal-to-Noise ratio measured on bin l during Initialization. The term, int[x] is defined as the highest integer lower than x, and bmin and bmax are minimum and maximum number of bits per bin supported.
In ADSL modems, on short loops and with little cross-talk impairment, performance is usually limited by imperfect equalization. Specifically, it is limited by Inter-Symbol Interference (ISI) and Inter-Channel Interference (ICI) terms. These terms are signal level dependent, so that if the power of the transmit signal is reduced, the ICI and ISI terms are reduced by the same amount. Thus, for such loops, if the modem is transmitting at standard power levels, power is being over consumed. Thus, there is a need for a method to reduce the power consumption in these circumstances.
It is an object of the present invention to obviate and mitigate at least some of the above mentioned disadvantages.